Question { 15929 }

difference between transducer and converter in instrumentation

Answer

Pretty much the same thing since both convert one type of
energy into a proportional different type of energy. Like
the pressure cell inside a pressure transmitter which we
call a transducer or sensor and a I/P converter which we
call a converter. Other examples are a analog to digital
converters or the sensors in a car's engine which are also
called transducers since they are stand alone and their
transmitters or monitors are in a different place.
Good luck

Question { Jindal, 26718 }

why the steam drum and deaerator level transmitter range is
in negative MMWC to zero

Answer

In both cases you are using a wet leg configuration with the
DPT. In the steam drum if you use the old fashion way by
changing the HP and LP legs around you will get a positive
mmH2O reading. If you use a piped SMART Tx or a SMART
capillary type DPT, you will get this negative mmH2O readings.
I have already done a writeup on this so go and look at
previous questions.
In the deaerator there is a vacuum but again the same wet
leg is done. I have already done a writeup on this vacuum
applications as well so go and look at previous questions.
Good luck

Question { 8795 }

in the boiler in the main steam line and its bypass line and
the startup vent vales are only motorised valves why cant we
use other types of valves such as pneumatic.

Answer

The design engineers must have had a reason why they
selected to use motorized valves in stead of pneumatic
valves. It might be that they felt the main power is less
likely to fall away than the plant air compressor tripping
and there is no air to operate these valves. It also
sometimes happens that the main instrumentation supply UPS
(24VDC) will trip and the whole plant looses the
instrumentation supply but the main supply is still available.
Also if the main power do fall away there is always the
emergency generators that will kick in and supply emergency
main supply to all the critical systems.
Just for interest they tend to select motorized valves for
safety systems as well like foam, fire pump control valves
and deluge main control valves, most probably for the same
reasons.
Good luck

Question { IOCL, 10425 }

without a positionar can a control valve operate? give the
difference of operating with positionar and without positionar?

Answer

There are applications where the positioner is actually
causing the process to run away, due to the valve reacting
being too fast, and the control will be much easier without
the positioner. You can do this by installing a I/P
converter, piped directly to the actuator.

In 99,99% of applications however, the positioner is
necessary to control the process since process changes is
normally very fast and smooth control will be impossible
without a positioner to speed up the reaction time of the
control valve and keeping the process on the desired control
set-point.
In these applications if you do not install a positioner you
will find that your process runs in a big sinuous wave form
and this will obviously cause a major chain reactions
through out the whole plant.
Good luck

Question { IOCL, 8687 }

in a electronic controller why we are using 250 ohm
resistance in input signal?is i/p is necessary in control
valve when we are using electronic controller give details?

Answer

The hardware electronic controller you are using has a 1-5
Volt input but the transmitter you use in the field, is a 4
to 20 mA transmitter. So in order for the hardware
controller to accept this input, you install a 250 Ohm
resistor on the controller's input which will convert it to
a 4 to 20mA input.

Weather you need a I/P converter on the control valve
depends on the positioner you will use. The hardware
electronic controller will give a 4 to 20mA signal to the
control valve, so if you want to use a full pneumatic
positioner on the valve, you need to install a I/P converter
and calibrate it for a 4 to 20mA input and a 20 to 100 Kpa
pressure output. You then need to calibrate the pneumatic
positioner for a 20 to 100 Kpa = 0 to 100% stroke.
If you want to use any other type of positioner a I/P will
not be necessary, since in electromechanical positioners the
I/P is already part of the positioner and obviously in SMART
positioners, the input to the positioner is 4 to 20 mA.
Good luck

Question { IOCL, 7411 }

what is interlock ckt? why it is necessary in industrial
machines ?

Answer

Not sure what you refer to with"ckt" but interlocks is all
about starting a application or machine safely.
For example if you want to start up a compressor the
discharge valve must be closed, the recycle valve must be
open, the guide vanes must be at 10%, there must be power
available, there must be air available, the oil must be at
60 degrees, the cooling water recycle pump must be running
and so on. The local control system(PLC) will only allow you
to start the compressor when all these interlocks have been met.
Good luck

Question { 4202 }

During valve sizing from all calculation you got flow
coefficient CV 12,than what it means? like for selecting
pressure transmitter if your MAWP is 12 bar than you select
a capsule of max. 20 bar which cover your mawp 12 in same
way for selecting control valve If CV is 12 than how we use
it in selecting valve??????

Answer

Go to the valve manufacturer's manual and select a valve
with the next bigger cv that the one you calculated.
Good luck

Question { 4617 }

hydrogen purity meter calibrating and commissioning
procedure can be explained?

Answer

It is easy. Find a manual and read it, that's what we all do.
The manufacturer can give you all the details of a specific
instrument much better that what anyone else can.
Good luck

Question { 4796 }

Could you explain to me the enraf (servo gauge) mechanism.
How it works ? and how it is working on archimedes theory
while it's not immersed into the surface? and how can we
calibrate it ? Thanx

Answer

It is easy. Find a manual and read it, that's what we all do.
The manufacturer can give you all the details of a specific
instrument much better that what anyone else can although it
seems that you already know quite a bit about it. Where did
you find that information? From the manual maybe, or are you
a scientist that knows about Archimedes theory as part of
you general knowledge about physics?
Good Luck

Question { 6412 }

Why Differential pressure transmitter is located below the
tappings points for steam & condensible vapor flow
measurement application?

Answer

Unfortunately I don't have a explanation for this one, since
I cannot see the reason for it either.
The standard on steam flow is to use condensation pots as
well, which are to me a bit of a waste of money, time and
effort since these pots have to be certified pressure
vessels, the same as any other pressure vessel on the plant.
I am not sure how many people realize this.

On wet leg levels, and in a case like this, I would rather
use a 1/2" S/S T-piece instead of a condensation pot, it
does the job just as well and it is certified and rated the
same as the S/S impulse lines.

I think the principle behind this whole steam flow
installation was based on what we normally do in the level
application of a pressurized steam drum. Since steam will
condense, we use this fact to our advantage by doing a wet
leg on it. The steam that condenses then make sure the wet
leg stays filled to the same level all the time.

In steam flow it seems like they have tried the same
principle, but according to me it is not necessary, since
the pressure taps will be installed on top of the flanges,
as for normal gas flow application, and not on the sides so
there is NO chance of liquid build up inside the transmitter
or the impulse lines, which are also installed above the
orifice plate. Any condensed liquid will immediately run
back into the line.
Doing the condensation pot installation and installing the
DPT below the line works obviously very well and is a very
good idea, I just cannot see why this is the standard or why
it should be done either.

Anyway it is not up to me or you to chance the international
instrumentation installation standards, so for now we have
to stick to this standard, but it is interesting to question
some of these original standards sometimes.
Good luck

Question { 16413 }

how to connect 3 wire 4-20 mA transmitter in Plc system PLC
system and how explain how the measuring data communicate
with plc system.

Answer

We normally use a clean separate power source to power all
the instrumentation on the plant including the DCS and
PLC's. This instrumentation power source is going through a
UPS and a inverter backup system to smooth and minimize
power failures. Each zone in the plant normally have its own
24VDC power supply unit, but they are all backed up by the
same UPS.
The UPS can normally keep the instrumentation and other
emergency and critical power for about 6 hours in case of a
total main power failure.

In your 3 wire system the supply to your TX will be from
this external power source, and the loop power for your 4 to
20 ma will be generated by the same TX and will not come
from the PLC or DCS as with a loop power 2-wire systems.
Since the external power to your TX is the same as the 24VDC
power it generates for the signal, you can use the negative
as a common. This common(OV) and the 4 to 20mA(24VDC) signal
is then connected to the PLC as for a normal 2-wire loop
powered TX.
This common from your TX is 0V and is the same as the 0V of
the PLC or DCS since they are all from the same UPS.

The PLC or DCS will only get the positive of your TX as the
input signal, since all the negatives of the inputs are all
connected together on a common 0V bar close to the the PLC
or DCS and only one wire from this negatives bar is then
daisy chained to each input. The negative from your TX you
therefore also need to connect to this common negative bar
of the PLC or DCS and only the positive goes to the input
connection.
The only thing you need to remember is that the 3 and 4 wire
systems are externally powered systems and are therefore
active inputs or source inputs and that the loop powered 2
wire system is a passive or sink type input. This you then
need to set up inside the PLC or DCS input configuration
parameters.
Good luck

Question { 5957 }

how can we measure the pressure in underground cooling
water pipeline by using pressure transmitter?what about the
tapping position?

Answer

You can install the PT anywhere on the pipe. It is best to
install on top in crude and slurry application but to
measure water you can install any where, even under the pipe.
If you install on the side or under the pipe fill the
impulse line with water then do a zero on the PT before you
do the final span calibration. Also remember that the
calibration you do on the PT must be exactly the same as to
what your DCS or SCADA faceplate have been scaled to. In
other words if you calibrate the PT for 0 to 20Bar you must
make sure the faceplate in the CCR are also scaled to 0 to
20bar.
Good luck

Question { 5797 }

IS IT POSSIBLE TO CALIBRATE A LEVEL TRANSMITTER INDICATOR(ABB)
USING HART COMMUNICATOR.? I MEAN TO CHANGE FROM ONE UNIT TO
ANOTHER WITHIN INDICATOR (EG. INCH TO FEET)...? PLZ NOTE
THAT I HAVE MADE THE UNIT CHANGE IN TRANSMITTER USING HART
COMMUNICATOR..BUT WHAT ABOUT THE INDICATOR..?

Answer

Ok I think I understand what you are asking. In most SMART
transmitters you have a digital indication on the
transmitter and you want to change what is displayed on this
indicator. You also said that you have changes the unit of
measure inside the transmitter with the HART, and this also
changed the UOM that is displayed.

There is a different parameter that you need to modify to
change the display. The parameter is normally something like
"set disply" or something like that.
Be aware that it will only display UOM or percentage or
display a switching between the two. What ever you have set
your UOM to inside the transmitter, will be displayed on the
indicator as the UOM. You cannot set the UOM to mmH2O inside
the transmitter and then display mBar on the display.
Also note that for a level application this display is
normally set to percentage and not UOM. If the operator look
at it he needs to see % level and not mm level. In a
pressure, flow or temp applications you want this display to
indicate UOM again.
Good luck

Question { 3065 }

differentiate different units of pressure like US & UK
standards

Answer

There are some difference in some areas like "area" in sq
feet and tons of force but nothing that concern us in
instrumentation. PSI is PSI all over the world and so is all
other units of measure we use in instrumentation. The UOM
that are slightly different between US and UK is not of any
importance to us.

All of these difference are also in the imperial system and
there are no differences when you work in the metric system.

In the area of pressure specifically, there is only the ton
of force that are only 1,2 more than the other and that is
not something you will ever use. The same is true for all
other UOM for temp, flow, level, density and all the ones we
use.
Good luck

Question { 14690 }

What's the Installtion procedure of DP Transmitter for
Gas,steam and water?

Answer

Gas(non condensing like hydro carbon gasses and air) -
Impulse line comes from pressure taps installed on top of
the flanges or pipe. The DPT is also installed on top of the
pipe so that any liquid that form will run back into the
main line via the impulse lines. It is best to work from a
process zero if the line pressure is above 5Bar.

Liquid - The DPT is installed below the pipe from impulse
lines installed on the sides of the flanges or pipe and
sloping down at a soft angle to the DPT. It is preferred not
to have any sharp bends in these liquid impulse lines. The
idea is to keep these impulse lines filled all the time and
that pressure differences to be transferred as directly as
possible. Keep in mind that in flow, the pressure
differences are sometimes only a few hundred pascals.
You also need to make sure these impulse lines are filled
before you do a zero calibration. The best zero you can do
is when the lines is completely filled but there is no flow
at all, but most of the time you will only be able to do a
zero by isolating the impulse lines from the pipe and then
do the zero.
It is best to work from a process zero if the line pressure
is above 5Bar. Keep the impulse lines filled during the
process zero. (See below)

Steam - The DPT is installed below the pipe from impulse
lines coming from the top of the flanges or pipe. There are
condensation pots (or T-pieces with a needle valve pointing
up)installed at the highest point and they should be at
exactly the same height.
From the bottom of these pots (or T-Pieces) the impulse
lines run down to past and below the DPT and ends with a
small valve at each end for draining purposes. The DPT is
connected with T-Pieces to these impulse lines.
The idea is to keep the condensation pots filled to the
overflow level all the time with hot water so that if any
condensation takes place the pot will just overflow back
into the main line and the different pressures on the DPT
will stay constant. In other words the zero stays constant.
It is best to work from a process zero if the line pressure
is above 5Bar. Keep the impulse lines filled during the
process zero. (See below)
Note:
Impulse line are never installed at the bottom of the
flanges or pipe since any flowing material will normally
deposit some stuff at the bottom inside of the pipe and this
could run into the impulse lines and block it or cause
density changes.

The flow process zero:
It all cases is best to do a process zero if the normal
flowing line pressure is above 5Bar. To do this on line
isolate both the impulse lines and remove the upstream
impulse line from the pressure tapping on the flange or
pipe. Install a flexible tubing with a T-Piece in it, to the
upstream pressure tapping. Connect this flexible tubing to
both sides of the DPT and close the equalization valve on
the manifold.
Open the isolation valve on the pressure tapping so that the
upstream line pressure is now on both sides of the DPT.
Most of the trapped air will flow up the flexible tubing
back into the main line but to be sure you can vent the DPT
at the manifold as well.
Do a zero trim and set the span value.
To make a more permanent installation, install two T-Pieces
in each impulse line just before the manifold and install a
needle valve between the two impulse lines.
Good luck